Surface acoustic wave convolvers or correlators are known as a small-scaled device which utilizes a surface acoustic wave for signal processing. They are generally classified into one type having monolithic structures and the other type having non-monolithic structures. Monolithic structures are more advantageous in manufacturing and effectiveness of these devices. In a monolithic surface accoustic wave convolver or correlator comprising a semiconductor substrate and a piezoelectric film, its signal processing function is caused by a non-linear interaction between a surface acoustic wave and a space charge region along a surface of the semiconductor. A structure which has been conventionally proposed to use the phenomenon is shown in FIGS. 9 and 10 wherein an insulating film 3 is provided on one surface of a semiconductor substrate 1, and a piezoelectric film 2 is provided on the insulating film 3. On the piezoelectric film 2 and near both ends thereof are provided input transducers 4a-4b for entering a signal thereinto and a gate electrode 5 for outputting a processed signal. FIG. 9 also shows a ground or bottom electrode 6 provided along the other surface of the semiconductor substrate 1. FIG. 10 further shows a variable d.c. bias source 7, a d.c. blocking capacitor 8, matching circuits 9a-9b-9c, signal sources 10a-10b, and an external load resistance 11 from which an output signal is extracted.
With this arrangement, non-linear interaction takes place just below the gate electrode 5 (this region is referred to as "interaction region" in this text), and an output is picked up from a point between the gate electrode 5 and bottom electrode 6. The magnitude of the interaction depends on the capacity-to-voltage (C-V) characteristic in the interaction region along the surface of the semiconductor substrate 1, and greatly varies with d.c. bias voltage applied between the gate electrode 5 and the bottom electrode 6 connected to ground. Therefore, it has been most usual in the prior art to uniformly apply to the entire interaction region a single bias voltage maximixing the total output from the gate electrode.
However, since the C-V characteristic of the interaction region is not uniform but varies with location, a uniform bias cannot be the best voltage value for some locations with different C-V characteristics, and cannot cause the optimum function of the device. This is particularly serious in a device having an elongated interaction region for an improved signal processing capacity.
Montress U.S. Pat. No. 4,328,473 and Minagawa U.S. Pat. No. 4,473,767 disclose technologies corresponding to the aforegoing prior art. In particular, Montress U.S. Pat. No. 4,328,473 discloses multiple bias applying gate electrodes and a single output gate electrode. However, since the output gate electrode is unitary or integral on the substrate, the bias applying gate electrodes and the output gate electrode are separate members. Therefore, although multiple bias sources are shown, they do not contribute to solution of the aforementioned problem.
Minagawa U.S. Pat. No. 4,473,767 teaches the use of a SAW device having a bias gate electrode. However, the bias gate electrode is used for adding alternating components of both transducers to a d.c. bias component. Therefore, although a bias is applied, no non-linear interaction occurs just under the bias gate electrode (this occurs in a separate semiconductor), and the output electrode of the device is a member separate from the gate electrode. This means that the device is far from a solution of the aforementioned problem.